Such products are produced by moulding thermoplastic resins, to which a blowing agent has been added. The pressure in the extruder of the injection moulding machine is maintained high enough to prevent any foaming due to dissociation of the blowing agent during the plastification of the molding material.
When the plastified material enters the mold, the blowing agent is decomposed and/or evaporated, forming a gas phase which is distributed as bubbles in the interior of the molded body, while a compact skin is formed on its surface.
Several methods for the production of such molded bodies are known. In one of them, a fixed portion of molten material is "shot"into the mold. This portion is about 50 to 90 percent of the volume of the mold cavity, depending on the desired grade of porosity. The cavity is then totally filled as a result of foaming. It is a drawback of this method, that, regardless of the speed of injecting the material into the mold, the surface of the molded bodies obtained is rough. This is due to the surface bubbles, however small they may be, which are formed immediately after opening the sprue channel towards the mold, and are located along the front of the melt entering the mold.
In another method the mold is filled initially totally with plastified material. After the formation of a compact skin, the mold is opened to a predetermined distance, providing thus an additional volume needed for foaming. Without avoiding the shortcomings of the first method, this technique can find a limited application only for components with a shape permitting the construction of such a mold.
The shortcomings of these methods can be avoided by using the socalled gas counter-pressure method. In this method a gas pressure, referred to as counter-pressure, is produced in the mold before the melt has been introduced into it. The melt enters the mold overcoming this pressure, until it fills it totally. The pressure is released after the formation of a solid skin, so as to provide conditions enabling foaming of the thermoplastic in the interior of the molding mass. The excess expanded material agresses through the sprue. It is thus possible to obtain smooth surfaces and to control the processes of skin formation and foaming.
One such method is known in which the excess expanded material is returned to the front zone of the extruder barrel, to be used in the subsequent injection. However, it is obvious that the retraction of the injection screw for providing the space needed for the excess core foam, will inevitably cause a drop of the pressure in the extruder barrel, which will result in foaming of all of the material contained in this barrel. In the subsequent injection this foamed material will be the first to enter the mold and the result will be an impairment of the quality of the skin. This is an important shortcoming, since laboratory trials have shown that, if premature foaming occurs, further increases in melt pressure will not result in a complete solution of the gas into the melt. A further shortcoming of this method is the complicated mold construction which requires channels in which pistons, driven by hydraulic units, move.